In a certain type of scanner where a mother document sheet moves to be read, the sheet sent out from a stack of sheets kept in a hopper is scanned while it passes across a reading glass plate provided in a reading line. The images on a mother document sheet are read out by an optical scanning system to be converted into electronic data.
Since the sheets are transferred at a relatively high speed, paper dust and fine toner particles are readily generated therefrom and disperse, such fine dust are attracted electro-statically onto the surface of the reading glass plate and the platen roller guiding the sheets. When the sheets are scanned for reading, the particles and dust ill-affect the image quality substantially, resulting in disturbing black lines or white-outs on a scanned image.
An image reader is provided with a sheet feeder for feeding sheets stacked in a hopper to the reading section. The sheet feeder picks up a sheet for delivery, one by one from the uppermost sheet of the stack. Most of the sheet feeders are equipped with a mechanism for preventing overlaid supply of sheets caused by mutual friction among the sheets. A sheet feeder having such a mechanism for preventing the overlaid sheet supply has been disclosed in Japanese Patent Laid-open Publication No. 4-286558.
FIG. 20 shows a schematic concept of a conventional sheet feeder. The sheet feeder as disclosed in the above-described laid-open patent is structured almost the same.
Referring to FIG. 20, a hopper 10 for holding mother document sheets or the like sheets P is provided at the reference end of a reading line, and a supply roller 20 is provided just above the hopper 10 for picking up the uppermost sheet for delivery. The hopper 10 is pushed upward by the force of a spring 30 so that the stack of sheets P is pressed to make contact with the supply roller 20. Because of friction with the roller surface, only a sheet located at the uppermost layer of the stack is sent out and delivered. Even when the stacking height of the sheets P varies, the spring 30 maintains the pressing force towards the supply roller 20 at substantially the same level.
At the outgoing side of the hopper 10, transfer rollers are provided, for example, in three stages 40, 50 and 60, for carrying the sheet P to the reading position. The sheet P coming out of the hopper 10 is nipped and pulled by the rollers for transfer. Between the hopper 10 and the first stage transfer rollers 40, a separation roller 70 and a retardation roller 80 are provided as means for preventing overlaid sheet transfer.
The overlaid sheet transfer prevention mechanism formed of the rollers 70 and 80 has been known among the image readers and the copying machines. The roller 80 is attached via a torque limiter 100 around a main shaft 90, which is driven by a driving motor (not shown) in a direction as indicated with an arrow in the drawing. The main shaft 90 is coupled with a driving motor (not shown), which is shared in common with the roller 70. The main shaft 90 is pushed upward (in FIG. 20) by a spring, in order to provide a nipping force in combination with the roller 70. The torque limiter 100 works as such: when one of the sheets P is sent out from the supply roller 20, the roller 80 revolves in compliance with the revolving torque of the roller 70 to carry the sheet forward; whereas, when two or more overlaid sheets P are sent out, the roller 80 revolves in the direction as indicated with an arrow to push the lower of the overlaid sheets back to the hopper 10.
In the above-described conventional mechanism, however, the supply roller 20 always stays on the stacked sheets P held in the hopper 10 and maintains contact thereon. Such an arrangement may impair the full functioning of the anti-overlaid transfer mechanism formed with the roller 70 and the roller 80. Namely, if the supply roller 20 is always in contact with the sheet P, it narrows the functioning range of the anti-overlaid transfer performance implemented by taking advantage of the relative relationship between the pressing force of roller 80 and the slip torque. It is difficult for the conventional mechanism to provide an optimized anti-overlaid transfer operation; as the result, the reliability in the prevention of overlaid sheet transfer is limited.
Furthermore, the conventional roller 80 uses a torque limiter 100 of fixed torque value, for which the operating torque cannot be changed.
Meanwhile, various kinds of document sheets come to the image scanners or the like devices; the sheets widely varying in the material, the thickness and the friction coefficient.
The sheet to sheet friction coefficient μ ranges approximately from 0.3-0.7. The high μ papers are liable to cause the overlaid sheet transfer. In order to prevent the overlaid sheet transfer with the high μ papers, the roller 80 needs to have a high retardation torque and the contact pressure also needs to be high. However, the roller 80 provided with the above-described torque and pressure is likely to induce overlaid sheet transfer with the low μ papers, and the papers might suffer from substantial damage.
Thus, the torque limiter 100, whose operating torque is fixed and invariable, may not be able to provide a sufficient anti-overlaid sheet transfer mechanism, working in combination with the roller 70. If an overlaid sheet transfer happens even once in an image scanner, where a number of mother document sheets are read out into the form of electronic data, the data are stored as they are and the stored data remain incomplete and defective. Thus the overlaid sheet transfer is a serious problem to be solved.
The present invention aims to offer an image reader, in which the reading glass plate and the platen roller are prevented from attracting paper dust and toner particles in order to read out quality images without being influenced thereby.
The present invention also aims to offer a sheet feeder used for image reading devices, which eliminates the overlaid transfer of document sheets for a number of sheets.